As described in J. Am. Chem. Soc., vol. 57, page 1007 (1935), monomolecular or multi-layered films prepared by the Langumuir-Blodgett method have the nature of ultra-thin films or two-dimensional crystals in which organic molecules are densely packed in a one-dimensionally oriented manner. Because of the capabilities provided by their characteristic nature, such ultra-thin films or two-dimensional crystals have been extensively used in practice as molecular devices.
The monomolecular film forming compounds which are suitable for the Langumuir-Blodgett method are amphoteric molecules, or molecules of the surfactant type which possess both hydrophilic and hydrophobic groups simultaneously. Commonly used amphoteric molecules are those having, as hydrophilic groups, organic groups or metal chelates containing acids, alcohols, esters, ethers, amines, etc. and having as hydrophobic groups, hydrophobic long-chain alkyl groups. When these amphoteric compounds as dissolved in organic solvents are spread over the surface of water, their molecules will diffuse over the liquid surface and become oriented to form a monomolecular film with the hydrophilic groups directed into water with the hydrophobic groups facing in the opposite direction. If this monomolecular film is compressed in a two-dimensional plane, it passes through a stage where it forms a two-dimensional liquid body, and thereafter the molecules are closely packed to form a stable solid film.
The formation of a stable monomolecular film in which the molecules are arranged in an orderly fashion is only possible with a molecular structure of the surfactant type described above. In the case of a non-surfactant type structure in which all of the molecules are hydrophobic, the molecules cannot be arranged in a desired way without causing agglomeration and it is difficult to provide a stable surface pressure in the structure. It is, therefore, important from a standpoint of molecular design of a monomolecular film forming compound that the compound be well balanced between the hydrophilic and hydrophobic groups and that it should be water-insoluble and non-volatile.
One of the features of such a monomolecular film that distinguish it from a so-called amorphous cast type film is that one surface of the monomolecular film is entirely occupied by hydrophobic groups while the other surface is entirely occupied by hydrophilic groups and that these groups are present at high density in each respective surface plane of the film. By utilizing this feature, the surface of a hydrophilic or hydrophobic substrate can be uniformly covered with such a monomolecular film with the hydrophilic groups being adsorbed on a hydrophilic substrate or the hydrophobic groups adsorbed on a hydrophobic substrate. If desired, a plurality of monomolecular films can be alternately built up on substrates by utilizing such molecular adsorption between hydrophilic groups or between hydrophobic groups. Such a resulting multi-layered film may also be characterized as an organic membrane in which the surface of its topmost layer is entirely covered with exposed hydrophilic or hydrophobic groups which are arranged at high density.
Methods are widely known in which a dissimilar guest compound is embedded in the surface or the interior of such a thin organic membrane having the construction described above and which supports the guest compound by utilizing the hydrophilic-hydrophilic and/or hydrophobichydrophobic interactions. In one of these methods, a dissimilar guest compound is mixed with a host compound in a solvent prior to film formation and the solution is then thinly spread to form a mixed monomolecular film. In this method, the guest compound is located between host compounds which serve as spacer molecules, so the amount of surface space occupied by the guest compound is far smaller than unity and the surface concentration of the guest compound is limited to a very low value. In another method, a monomolecular film of host molecules is first formed on the surface of water or a substrate and thereafter guest molecules are supplied from the aqueous phase (subphase) side in such a way that they are attached to, or incorporated in, the monomolecular film by means of diffusion and adsorption. In this case, the adsorption of the guest molecules is generally accomplished by the hydrophobic binding force or the coulomb force of attraction. Fromherz et al. suggested that the latter method can be used to incorporate a bioactive enzyme (e.g. trypsin) as a guest molecule in a host monomolecular film of an aliphatic acid or an aliphatic acid ester by means of adsorption (as described, e.g., in FEBS Letters, vol. 49, page 329 (1975)). The latter method can also be used to adsorb a viable protein on a monomolecular film which is subsequently applied to cover a substrate, as described in Japanese Patent Application (OPI) No. 251930/85 (FR 8407213) (the term "OPI" used herein means a published unexamined Japanese patent application). However, the monomolecular films in which functional guest molecules are supported by these adsorption or complex-forming techniques have the disadvantage that the guest molecules are readily detached from the film by washing with water or by other factors since they are supported on the monomolecular films merely by means of chemical equilibria of adsorption or complex association. If a chemical reaction is to be carried out on the monomolecular film by utilizing the catalytic activity or other capabilities of the guest molecules, a great disadvantage in practical application arises, such as lowered reactivity due to the desorption of the guest molecules during reaction. The above methods have the additional disadvantage that the capabilities of the guest molecules are not fully exhibited because the functional groups in the guest molecules are either randomly oriented on the surface of the monomolecular film or buried between the host molecules in the monomolecular film.